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Pilot's Handbook of Aeronautical Knowledge
Weather Theory

Measurement of Atmosphere Pressure

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Atmosphere weights.
Figure 11-4. Atmosphere weights.

Coriolis Force
In general atmospheric circulation theory, areas of low
pressure exist over the equatorial regions and areas of high
pressure exist over the polar regions due to a difference in
temperature. The resulting low pressure allows the high pressure
air at the poles to flow along the planet's surface toward the
equator. While this pattern of air circulation is
correct in theory, the circulation of air is modified by several
forces, the most important of which is the rotation of the

The force created by the rotation of the Earth is known as
the Coriolis force. This force is not perceptible to humans as
they walk around because humans move slowly and travel
relatively short distances compared to the size and rotation
rate of the Earth. However, the Coriolis force significantly
affects bodies that move over great distances, such as an air
mass or body of water.

The Coriolis force deflects air to the right in the Northern
Hemisphere, causing it to follow a curved path instead of a
straight line. The amount of deflection differs depending on
the latitude. It is greatest at the poles, and diminishes to zero
at the equator. The magnitude of Coriolis force also differs
with the speed of the moving body—the greater the speed,
the greater the deviation. In the Northern Hemisphere, the
rotation of the Earth deflects moving air to the right and
changes the general circulation pattern of the air.

Three-cell circulation pattern due to the rotation of the Earth.
Figure 11-5. Three-cell circulation pattern due to the rotation of
the Earth.

The speed of the Earth's rotation causes the general flow
to break up into three distinct cells in each hemisphere.
[Figure 11-5] In the Northern Hemisphere, the warm air at
the equator rises upward from the surface, travels northward,
and is deflected eastward by the rotation of the Earth. By
the time it has traveled one-third of the distance from the
equator to the North Pole, it is no longer moving northward,
but eastward. This air cools and sinks in a belt-like area at
about 30° latitude, creating an area of high pressure as it
sinks toward the surface. Then, it flows southward along
the surface back toward the equator. Coriolis force bends
the .ow to the right, thus creating the northeasterly trade
winds that prevail from 30° latitude to the equator. Similar
forces create circulation cells that encircle the Earth between
30° and 60° latitude, and between 60° and the poles. This
circulation pattern results in the prevailing westerly winds
in the conterminous United States.

Circulation patterns are further complicated by seasonal
changes, differences between the surfaces of continents and
oceans, and other factors such as frictional forces caused
by the topography of the Earth's surface which modify the
movement of the air in the atmosphere. For example, within
2,000 feet of the ground, the friction between the surface and
the atmosphere slows the moving air. The wind is diverted from
its path because the frictional force reduces the Coriolis force.
Thus, the wind direction at the surface varies somewhat from
the wind direction just a few thousand feet above the Earth.

Measurement of Atmosphere Pressure

Atmospheric pressure is typically measured in inches of
mercury ("Hg) by a mercurial barometer. [Figure 11-6] The
barometer measures the height of a column of mercury inside a
glass tube. A section of the mercury is exposed to the pressure
of the atmosphere, which exerts a force on the mercury. An
increase in pressure forces the mercury to rise inside the tube.
When the pressure drops, mercury drains out of the tube,
decreasing the height of the column. This type of barometer is
typically used in a laboratory or weather observation station,
is not easily transported, and difficult to read.